The present invention relates to machines which inspect glass bottles for defects which could have occurred in the forming process. A light source directs light through the sidewalls of the bottle and imperfections will interfere with the light rays. For example, should a wall have a blister, the blister may change the direction of the light rays passing therethrough and as a result these rays may not reach the viewing medium such as a CCD camera causing the blister to look black. Such blackness would indicate a defect.
Critical to the inspection process is the comparison of adjacent pixels in the camera by a computer which compares the light intensity of these pixels. When adjacent pixels progress from light to dark, the computer recognizes that there is an edge and its evaluation may define the edge as the edge of a defect.
Some bottle surfaces are formed with stippling, (lots of small bumps) on the surface (stippling can be generalized to various kinds of embossing). Stippling appears as grey spots in the image, which when edge detected is detected as a significant enough change to mask defects even though they may be darker or have sharper edges. The grey spots are the result of a loss of light getting to the camera because the stippling has acted like a lens to spread that light out and so only a part of the light passing through there gets to the camera. Around the stippling the light is not being dispersed and so appears brighter to the camera.